Conventionally, reflective pulse modulation photodetectors have been widely used which detect presence or absence of a physical object. In such reflective pulse modulation photodetectors, pulsed light is projected which has a pulse width modulated according to a light emission pulse generated based on a clock pulse signal and having a modulation cycle. Then, presence or absence of a physical object is detected according to whether or not the pulsed light reflected by the physical object passing in front of a light emitting element and a light receiving element is received. Further, transmissive pulse modulation photodetectors have also been widely used which detect presence or absence of a physical object according to whether pulsed light from the light emitting element is received by a light receiving element or blocked by the physical object passing between a light emitting element and the light receiving element.
FIG. 10 is a block diagram illustrating a structure of a conventional photodetector 900. FIG. 11 is a block diagram illustrating a specific structure of the photodetector 900 including a light emission pulse generating circuit 903 and a signal processing circuit 914. FIG. 12 is a timing chart representing operations of the photodetector 900.
The photodetector 900 includes an oscillator circuit 902. The oscillator circuit 902 generates a clock pulse signal S901, and provides it to the light emission pulse generating circuit 903. The light emission pulse generating circuit 903 modulates the clock pulse signal S901 thus received from the oscillator circuit 902, so as to generate and provide a light emission pulse signal S902 to a light emitting element drive circuit 907. The light emission pulse signal S902 includes and a light emission pulse p901 having a modulation cycle t901 and a pulse width w903. The light emitting element drive circuit 907 drives a light emitting element 908 based on the light emission pulse signal S902 received from the light emission pulse generating circuit 903, causing the light emitting element 908 to project pulsed light 909 having a modulated pulse width.
The pulsed light 909 having a modulated pulse width is reflected by a physical object 910 passing in front of the light emitting element 908 and a light receiving element 911, and is incident on the light receiving element 911. Thus, if there is no ambient light in the vicinity, the pulsed light 909 directed to the light receiving element 911 is turned ON or OFF depending on whether or not the object A passes.
The light receiving element 911 photoelectrically converts the pulsed light 909 reflected by the physical object 910, so as to generate and provide a receiving light pulse signal S903 to an amplifier 912. The amplifier 912 amplifies the receiving light pulse signal S903 received from the light receiving element 911, and provides it to a determination circuit 913. The determination circuit 913 corrugates the receiving light pulse signal S903 thus amplified by the amplifier 912, so as to generate and provide a determination signal S904 to the signal processing circuit 914. The signal processing circuit 914 outputs a high or low level signal to an output circuit 919, based on the determination signal S904 received from the determination circuit 913.
The light emission pulse generating circuit 903 includes a 4-stage binary counter 904. The 4-stage binary counter 904 includes flip flops 905a, 905b, 905c, and 905d, which are connected in series. The light emission pulse generating circuit 903 further includes a timing circuit 906 which generates the light emission pulse signal S902 based on signals outputted from the 4-stage binary counter 904.
The signal processing circuit 914 includes a latch circuit 915, a state detection circuit 916, and a 3-stage shift register 917. The latch circuit 915 latches the determination signal S904 received from the determination circuit 913. Based on the light emission pulse signal S902 generated by the timing circuit 906, the state detection circuit 916 detects a state of a signal outputted from the latch circuit 915. Further, the 3-stage shift register 917 outputs a high or low level signal to the output circuit 919, based on the result of detection made by the state detection circuit 916. The 3-stage shift register 917 includes flip flops 918a, 918b, and 918c, which are connected in series.
The timing circuit 906 generates the light emission pulse signal S902 having one light emission timing (1/24) duty (1/16 duty: pulse width w903) in one cycle. As used herein, one cycle (i.e., modulation cycle t901) is constituted by 24. (i.e., 16) basic clocks. The light emission pulse signal S902 is produced from the combination of frequency dividing pulse signals Q905a, Q905a_, Q905b, Q905b_, Q905c, Q905c_, Q905d, and Q905d_, whose frequencies are divided by the flip flops 905a, 905b, 905c, and 905d of the 4-stage binary counter 904.
The timing circuit 906 provides the light emission pulse signal S902 to the light emitting element drive circuit 907. Further, the timing circuit 906 provides the light emission pulse signal S902 to the signal processing circuit 914 by using (i) as a synchronous timing, a time period which corresponds to the pulse width w903 and which indicates the light emission timing and (ii) as asynchronous timing, time periods other than the period corresponding to the light emission timing.
In a case where neither ambient light nor the pulsed light from the light emitting element 908 is incident on the light receiving element 911, the determination signal S904, provided from the determination circuit 913 to the signal processing circuit 914, indicates that no pulse signal is present in the synchronous timing of the determination signal S904. Thus, the state detection circuit 916 determines that there is no signal and no noise, and the flip flops 918a, 918b, and 918c of the 3-stage shift register 917 output shift register output signals Q918a, Q918b, and Q918c in a low level. As such, as long as no pulse signal is present in the synchronous timing of the determination signal S904 and there is no ambient light, the signal processing circuit 914 outputs a low level signal.
In a case where there is no ambient light and where the pulsed light emitted from the light emitting element 908 and reflected by the physical object 910 is incident on the light receiving element 911, the determination signal S904, provided from the determination circuit 913 to the signal processing circuit 914, indicates that a pulse signal is present in the synchronous timing of the determination signal S904. Therefore, the determination signal S904 is latched by the latch circuit 915, and the state detection circuit 916 determines that there is a signal and no noise. Accordingly, the shift register output signals Q918a, Q918b, and Q918c of the shift register 917 are inverted from low to high, while being transmitted through the three stages. As a result, the output circuit 919 receives the signal in a high level. As such, in the case where there is no ambient light and where a pulse signal is present in the synchronous timing of the determination signal S904, the signal processing circuit 914 outputs a high level signal.
Regardless of a demand for a reduction in consumption current of optical modulation detectors, the ratio of a current flowing through a light emitting element such as an LED to an overall consumption current of the optical modulation detector is large. On the other hand the S/N ratio of the light emitting elements should not be reduced in order that circuit malfunction does not easily occur even when light including noise components is incident on the light emitting element. Since a current of not less than a predetermined value needs to be flown through the light emitting element, it is difficult to reduce a consumption current of the light emitting element.
Further, in a case where a plurality of light emission timings are set in 1 modulation cycle to prevent (i) error detection of ambient light and (ii) unauthorized use of signal(s), a current flowing through a light emitting element such as an LED increases. This causes more difficulties in reducing a consumption current of the light emitting element.
Japanese Unexamined Patent Publication 209250/1994. (Tokukaihei 6-209250, publication date: Jul. 26, 1994) (Patent Document 1) discloses an arrangement in which a detection activating circuit detects whether or not a photoelectric switch senses an object etc. In the arrangement disclosed in the publication, if an object etc. may not be sensed, an oscillation cycle of an oscillator circuit is set to a long cycle T1, while the oscillation cycle is set to a short cycle T2 if an object etc. may be sensed.
In the arrangement disclosed in the publication, however, a consumption current is still large in a time period during which an object etc. may not be sensed. Thus, the consumption current cannot be reduced sufficiently.